Fly line to leader connection/knot

[dryflyguy]

pretty sure the first cheapy flyrod outfit i had a million years ago had a metal barbed eyelet inserted into the end of the line. i actually think i caught fish with it and it held.

I inherited a fly rod from a distant family member when I first started out.
And the fly line (level) came with one of those on it.
Thinking I may have actually used it a bit
And am wondering, if I did, how I attached the leader to it